1. Field of the Invention
The present invention relates to a wordline driver applied in a DRAM, and more particularly to a power-saving wordline driver.
2. Description of the Related Art
DRAM is a commonly-seen memory, which is widely used in computers and other electronic appliances. Generally, DRAM includes a memory cell array composed of capacitors to store data. Because the capacitors lose charges after a period of time, it is necessary to refresh data in a fixed interval.
During the process of writing data to DRAM, to write a logic one voltage to the memory unit, it is necessary to provide a wordline precharge voltage VPP that is greater than an external voltage to the wordline. To generate voltage VPP, it is unavoidable to employ a VPP charge pump. However, the driving efficiency (Ivpp/Ivext) of the VPP charge pump is so low that the power consumption of the wordline driver is hardly to reduce.
FIG. 1 shows a commonly-used wordline driver 10. When the wordline is charged from VNWL to voltage VPP, the signals WLRST, bMWL, wldint must be logic zero. Meanwhile the voltage VPP charges the wordline through transistors 11 and 12, therefore the necessary current charging the wordline is provided by the voltage VPP. As mentioned above, because the necessary current is provided by the voltage VPP, the power consumption is large.
FIG. 2 shows a corresponding timing diagram of the wordline driver 10, wherein after the control signal wldint turns to zero, the output voltage WLDV of the transistor 11 is close to voltage VPP. In addition, after bMWL turns to logic zero, the voltage of the wordline (the output of the transistor 12) is close to voltage VPP. Because the control signal WLRST is logic zero, the transistor 13 remains off and the voltage of the wordline will not decrease.
To improve the power consumption of the wordline driver, some known methods tend to increase the driving efficiency of the VPP charge pump as much as possible. Therefore it is an important issue to control the power consumption of the wordline driver without adding many hardware resources.